In a wavelength division multiplexing (WDM) in a trunk-line, a modulation speed and a transmission capacity have been increased. An optical modulator used in this system has been desired not only that it is driven with a low voltage and it is operated with high modulation speed, but also that it is compact, an optical modulating characteristic does not depend upon a wavelength, and it is compatible with a multilevel coding modulation, which has a high use efficiency of a band.
The optical modulating system that is currently put into practical use is classified, from a viewpoint of modulation principle, into a system directly modulating a light source (laser), an electro absorption optical modulator, and an optical modulator (Mach-Zehnder optical modulator) utilizing a Mach-Zehnder interferometer. Among these, the Mach-Zehnder (MZ) optical modulator can be used for a long-distance transmission such as a trunk-line of an optical modulator, and is excellent that a wavelength dependency of the optical modulating characteristic is small.
An LN optical modulator using lithium niobate (LiNbO3, hereinafter referred to as LN) has typically been used for the MZ optical modulator. However, since the LN is a dielectric material, an advanced fabrication process is needed for the LN optical modulator. Since the length of the device is relatively long, there is a problem in the reduction in size upon assembling in an optical communication system.
On the other hand, a semiconductor MZ optical modulator using a compound semiconductor can be made compact, and further, can be monolithically integrated with a light-emitting device. The semiconductor MZ optical modulator has a device layer structure in which, when an electric field is effectively applied to a core of an optical waveguide, a refractive index is changed so as to change a length of an effective optical path.
The semiconductor MZ optical modulator needs an optical waveguide suitable for a phase modulator. As the features of the optical waveguide suitable for the phase modulator, such examples are given from the viewpoint of the characteristic that the optical waveguide has a great change in the refractive index, a low loss, a single mode characteristic, and low bending loss, and that the optical waveguide is easy to be fabricated (has large tolerance) from the viewpoint of fabrication.
The structure of the waveguide is classified into a rib (ridge) type, a high-mesa type, and a buried type as an optical confinement type. FIGS. 10, 11, and 12 respectively illustrate a sectional view of a waveguide of the rib type, the high-mesa type, and the buried type. Numeral 901 denotes a first clad layer, 902 denotes a core layer, 903 denotes a second clad layer, and 904 denotes a current blocking region. The current blocking region 904 is a region having a function of a clad in the optical waveguide. The rib waveguide is excellent in that a process of fabricating a device can be simplified, since it does not need a regrowth process, and further, since the rib waveguide has a structure in which the core layer 902 is not exposed, the device reliability is not deteriorated even in a semiconductor material (for example, InGaAlAs material or the like on an InP substrate), which is liable to be oxidized. The rib waveguide has a feature of being unsusceptible to a roughness on an etching side face. Therefore, a semiconductor MZ optical modulator using a rib waveguide has been expected.
The rib waveguide is not limited to the structure in FIG. 10, but a structure of the waveguide in which the core layer 902 is partly removed as illustrated in FIG. 11 has been proposed (for example, Patent Document 1). An optical mode control in the rib waveguide in this case is performed with a width W of a rib portion of the second clad layer 903 as in the rib waveguide in FIG. 10. The Patent Document 1 describes that a structure, which can realize a high-speed response because the core layer 902 is partly removed to reduce a parasitic device, can be provided.
The Patent Document 2 describes an optical modulator including a low-temperature growth GaN buffer layer, n-GaN clad layer, non-dope InGaN optical waveguide layer, and p-GaN clad layer, those of which are successively stacked in this order on a (0001) surface sapphire substrate. It also describes that an etching is performed up to the portion above the non-dope InGaN optical waveguide layer by an etching process, whereby a ridge optical waveguide structure is fabricated. It also describes that, since a crystal having a wurtzite-type crystal structure has a strong ion binding property, a large piezoelectric effect, and an increased Pockels effect, a reduction in a voltage, a reduction in size, and an increased modulation speed can simultaneously be realized.
The Patent Document 3 describes an optical modulator in which a structure of the optical waveguide formed into a rib type is formed to satisfy a single waveguide mode condition, and a conductive second clad layer is removed in a region apart from the ridge shape to a degree of not affecting the waveguide mode condition. It describes that this structure can reduce a coupling loss, improve a DC reverse bias characteristic, and perform a high-speed modulation.
The Patent Document 4 describes a ridge optical waveguide structure including a portion that induces light with high intensity, a portion that is bonded by an intermediate portion for inducing light with low density, and the intermediate portion where a mode confinement is gradually changed between the portion inducing light with low intensity and the portion inducing light with high intensity. It also describes that, as the width of the intermediate portion decreases due to a taper, the confinement of the mode is changed so as to cause a squeezing, whereby a mode mismatch is reduced in the binding of the low-intensity inducing portion and the high-intensity inducing portion.